Conversion of doped polycrystalline material to single crystal material

ABSTRACT

A solid state method of converting a polycrystalline ceramic body to a single crystal body includes the steps of doping the polycrystalline ceramic material with a conversion-enhancing dopant and then heating the polycrystalline body at a selected temperature for a selected time sufficient to convert the polycrystalline body to a single crystal. The selected temperature is less than the melting temperature of the polycrystalline material and greater than about one-half the melting temperature of the material. In the conversion of polycrystalline alumina to single crystal alumina (sapphire), examples of conversion-enhancing dopants include cations having a +3 valence, such as chromium, gallium, and titanium. The polycrystalline body further can be inhomogeneously doped to form a first portion of the polycrystalline body that is doped to the selected level of the conversion-enhancing dopant and a second portion that is not doped such that heating the doped polycrystalline body causes conversion of first portion to a single crystal structure and the second portion retains a polycrystalline structure.

RELATED APPLICATIONS

This is a divisional application of U.S. Ser. No. 08/195,187; filed Feb.14, 1994 now U.S. Pat. No. 5,487,353.

This application is related to copending applications entitled "SolidState Formation of Sapphire Using a Localized Energy Source", Ser. No.08/064,386, filed 21 May 1993; and the following applications filed 24Sep. 1993: "Solid State Formation of Sapphire From PolycrystallineAlumina Using a Seed Crystal", Ser. No. 08/126,628; "Solid State ThermalConversion of Polycrystalline Alumina to Sapphire", Ser. No. 08/126,954;and "Conversion of Polycrystalline Material to Single Crystal MaterialUsing Bodies Having a Selected Surface Topography", Ser. No. 08/126,830,all of which are assigned to the assignee of the present invention andare incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a solid state process for bulk conversion of apolycrystalline ceramic body to a single crystal body by doping thepolycrystalline body with a selected dopant in a selected concentrationand then heating the doped body to temperatures above one-half themelting temperature of the material but below the melting temperature ofthe material. More particularly, this invention relates to a solid stateprocess for the bulk conversion of polycrystalline alumina (PCA) to asingle crystal alumina (sapphire). The solid state conversion of PCA tosapphire is accomplished by doping the alumina body with a selecteddopant in a selected concentration and then heating the doped aluminabody to temperatures above 1100° C. but below 2050° C., the meltingpoint of alumina.

BACKGROUND OF THE INVENTION

The manufacture of polycrystalline alumina (PCA), and its use for highpressure sodium arc discharge lamps (hereinafter "HPS lamps")is known inthe art. U.S. Pat. Nos. 3,026,210; 4,150,317 and 4,285,732 to Coble,Laska et al and Charles et al, disclose the production of a high densityPCA body having improved visible light transmission using relativelypure alumina powder and small amounts of magnesium oxide. U.S. Pat. No.4,285,732 further teaches adding zirconia and hafnia to the magnesiumoxide-doped alumina to reduce the chances of precipitating a spinelphase and exaggerated or run away grain growth during sintering. PCAbodies useful as arc tubes for HPS lamps have been made according to theprocesses in these patents having an average grain size ranging frombetween 15 microns to 100 microns.

Two major drawbacks associated with the use of PCA arc tubes for HPSlamps are that they are light translucent as opposed to lighttransparent and the sodium in the arc reacts with the alumina at thegrain boundaries to form sodium aluminate, which adversely affects thestructural integrity of the tube and shortens lamp life. HPS lamps arebeing designed for ever increasing internal sodium partial pressurewithin the PCA arc tube to improve the color rendition and provide awhiter emitted light. However, higher internal sodium pressure furthershortens lamp life due to increased rate of sodium loss from the arcchamber. Progressive sodium loss results in a corresponding continualrise in the lamp operating voltage, a decrease of both correlated colortemperature and color rendering index, and a color shift from white topink. Also, the sodium which migrates through the arc chamber walldeposits on the inside wall of the evacuated outer lamp envelope causinga brownish stain on the envelope which, in turn, further reduces thelight output of the lamp. These problems are substantially reduced withsapphire (i.e., single crystal alumina) arc tubes.

Sapphire arc tubes useful as the arc chamber for HPS lamps have beenmade by a number of processes, including a modified Czochralski processknown as the edge-defined, film-fed growth (EFG) process developed byTyco Laboratories, Inc. This process uses a seed crystal and a die onthe surface of molten alumina in which a hollow tube is continuouslypulled out of the melt through the die. This process is expensive andslow. Another process used to produce single crystal alumina (sapphire)is called the floating zone process in which a PCA feed rod isintroduced at a predetermined velocity into a heating zone wherein oneor more lasers or other concentrated heat source is focused on the rodto melt the alumina in the zone to produce a "melt volume" of moltenalumina. A sapphire fiber is continuously drawn from the melt volume atthe desired velocity and the feed rod is moved simultaneously, but at aslower rate so that the process is a continuous one. This process isused primarily to produce sapphire fibers and does not readily lenditself to production of single crystal alumina tubing, although its usefor such is disclosed in U.S. Pat. No. 3,943,324.

Japanese Patent Publication 62-28118 of H. Yoshida et al. discloses thatsapphire can be synthesized via a solid state process by inducing amagnesium oxide concentration gradient along the length of a PCA body toensure grain growth is initiated at a single point on the PCA bodyduring heat treatment. This magnesium oxide gradient can be produced inthe PCA body by doping the green body with magnesium oxide in such a waythat there is a magnesium oxide gradient in the PCA body, by using atemperature gradient to create the magnesium oxide concentrationgradient, or by thinning a section on the greed body. Key to the Yoshidaprocess is that the growth of the single crystal initiates from a singlelocation in the polycrystalline body. Further, it is not known if thisprocess was commercialized and the disclosure, taken at face value,appears to present potential difficulties in its implementation. Inparticular, Yoshida et al. require only the equivalent of 90 wppm(weight parts per million) magnesium oxide in their alumina startingbody. Yet, in order to realize a dense PCA structure, at least about 300wppm of magnesium oxide is required. See, e.g., J. G. J. Peelen"Alumina: Sintering and Optical Properties", Ph.D Thesis, TechnicalUniversity of Eindhovan, Netherlands May 1977. Typical aluminadensification processes, such as those used to manufacture Lucalox®brand PCA, have 550-750 wppm magnesium oxide in the alumina startingbody to ensure that full density is achieved. At 90 wppm of magnesiumoxide, a dense, pore-fee structure, specified by Yoshida et al. as theirstarting material, is not readily achievable.

A need exists for producing sapphire from PCA in a facile and relativelyinexpensive manner. It is desirable to fabricate ceramic objects havingsimple or complex shapes using standard polycrystalline formingtechniques and then convert the object into a single crystal bodywithout melting the body. Thus, the single crystal body maintains theshape of the polycrystalline precursor, enabling the fabrication of agreat diversity of shapes that are not feasible to fabricate usingconventional melt drawing or floating zone techniques. A solid stateconversion process would make it possible to manufacture single crystalarticles having non-uniform, asymmetric and complex shapes as well assimple shapes. It would also be a great improvement to the art if such aprocess were cost effective in greatly reducing both the energy and thetime required to effect the formation of a single crystal ceramicstructure from a polycrystalline ceramic structure.

SUMMARY OF THE INVENTION

In accordance with the present invention, a solid state process forconverting a dense polycrystalline body to a single crystal bodyincludes the steps of doping the polycrystalline material comprising theceramic body with a selected conversion-enhancing dopant and heating thepolycrystalline body at a selected temperature for a time sufficient tosubstantially convert the polycrystalline body to a single crystal body.As used herein, "Solid state process" refers to a process in which theconversion of the polycrystalline body to a single crystal occurs atemperature below the melting temperature of the material such that nopan of the polycrystalline body melts, and no molten zone is formed inthe body during the heating steps.

The selected temperature at which the body is heated is less than themelting temperature but greater than about one-half the meltingtemperature of the polycrystalline material. The polycrystallinematerial is doped with the selected conversion-enhancing dopant to aselected concentration such that the final dopant concentration in thesingle crystal body is not greater than the solid solubility of thedopant in the material. For example, selected dopants introduced intopolycrystalline alumina (PCA) bodies enhance conversion of the PCA tosapphire in accordance with the process of the present invention.Selected dopants shown to enhance conversion of PCA include materialscomprising cations having a +3 valence; examples of such materialsinclude chromium, gallium, and titanium; further, it is anticipated thatother materials that form such cations, such as cerium, lanthanum,samarium, and vanadium would similarly enhance such conversion.

Doping of the polycrystalline material can be accomplished in severalways. For example, the dopant is introduced into raw materials used toproduce the polycrystalline body; alternatively, the bisque-firedprecursor material of the polycrystalline body is immersed in a liquiddoping solution having a predetermined concentration of the dopant sothat the doping solution infiltrates the bisque-fired body. Infiltrationof the bisque-fired body by the doping solution is accelerated byexposing the immersed bisque-fired material to a vacuum.

The method of this invention is particularly adapted for the solid stateconversion of polycrystalline alumina (PCA) to sapphire. The selecteddopant is introduced into the bisque-fired alumina body, for example, byimmersing the bisque fired alumina body in the doping solution in avacuum chamber and drawing a selected vacuum on the chamber for aselected time. The doped alumina body is then heated; heating of thedoped alumina body sinters the bisque-fired body so as to form a densebody, substantially removes many conversion-impeding impurities, such asmagnesium oxide (the raised temperature drives off the impurities fromthe material comprising the ceramic body), and converts thepolycrystalline alumina to a single crystal structure (sapphire). Theheating step comprises a single heating process in which the body israised to a selected temperature and maintained at that temperature fora selected time, or alternatively, the heating may comprise separateheating cycles for sintering, removing conversion-impeding impurities,and converting the polycrystalline alumina to sapphire.

In an alternative embodiment, the starting polycrystalline body isinhomogeneously doped such that, on undergoing heat treatment inaccordance with this invention, the resulting heat-treated body is acomposite material having polycrystalline and single crystal regions (orportions). In this embodiment the polycrystalline body isinhomogeneously doped so as to have a first portion doped with theconversion-enhancing dopant to the selected concentration. A secondportion of the polycrystalline body is not doped, or alternatively,doped to a second phase level concentration of the dopant, the secondphase level concentration being greater than the solid solubility levelof the dopant in the polycrystalline material such that a second phaseis formed along grain boundaries of the second portion of the body so asto inhibit conversion of the polycrystalline material to single crystal.The heating of the inhomogeneously doped body produces a compositematerial body in which the first portion comprises a single crystalstructure and the second portion comprises a polycrystalline structure.

Sapphire produced according to the process of this invention isdistinguished from sapphire fabricated with more traditional meltprocesses by the combination of a random pore structure and a uniquesurface topography in the form of slight undulations having high pointsat approximately the midpoint of where each PCA grain was located priorto the conversion and depressions corresponding to the where the grainboundaries were located prior to conversion.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description in conjunction with the accompanying drawingsin which like characters represent like parts throughout the drawings,and in which:

FIG. 1 is a graph illustrating the relationship of dopant level and timeof conversion for selected conversion-enhancing dopants in accordancewith this invention.

FIG. 2 is a schematic illustration of a high pressure sodium dischargelamp having an arc tube fabricated in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a polycrystalline body isdoped to a selected concentration with a conversion-enhancing dopant andis then heated at a temperature that is less than the meltingtemperature, but greater than about one-half the melting temperature ofthe polycrystalline material, for a time sufficient to substantiallyconvert the polycrystalline body to a single crystal body.

A starting material which has been found useful in the practice of thisinvention, wherein the polycrystalline body comprises polycrystallinealumina (PCA) and the PCA body is converted to single crystal alumina(hereinafter sapphire), is a bisque-fired body of relatively pure alphaalumina having an interconnected pore structure. An interconnected porestructure facilitates uniform penetration of the doping solutionthroughout the bisque-fired body. The typical starting bisque-firedmaterial has a pore volume ranging from about 15-70%. in general, thestarting material is a bisque-fired body composed of 99.9% alumina whichcan be sintered to densities greater than 3.90 g/cc after doping and isfree of impurities of a type and amount that would prevent theconversion of the sintered PCA body to sapphire. When sintered, thebisque-fired body typically produces a PCA material having an equiaxedgrain structure with average grain sizes less than 100 microns andpreferably less than 70 microns. By grain size is meant the averagedimension of a grain as measured by the well known linear intercepttechnique described in ASTM E112-88. PCA materials with average grainsizes larger than 100 microns tend to form microcracks during the heattreatment of this invention which prevents the conversion to sapphire.The density of the sintered PCA should be at least 3.90 g/cc and moretypically greater than 3.97 g/cc as the residual porosity can impede theconversion to sapphire and/or yield a sapphire product having less thanoptimal light transmittance.

in the production of sapphire in accordance with this invention, thestarting bisque-fired material was bisque-fired alumina tubes used forthe manufacture of Lucalox® tubes which have outer diameters rangingfrom 4.5 mm to 8.8 mm and wall thicknesses ranging from 0.5 mm to 0.75mm. This bisque-fired material is available from General ElectricCompany, Willoughby Quartz and Ceramic Plant, Willoughby, Ohio (Product#LT5.5-36-PS; Resource #258 23 61). This bisque-fired material typicallyhas a pore volume of 50-60%. When sintered, this material produces a PCAbody having densities ranging from 3.97 g/cc to 3.98 g/cc and anequiaxed grain structure with average grain sizes ranging from 15-70microns. A typical trace impurity analysis for the sintered material(undoped) is given in Table 1 below. The 180 wppm concentration ofMagnesium (Mg) is equivalent to about 300 wppm magnesium oxide (MgO).

                  TABLE 1                                                         ______________________________________                                        Trace                                                                         Element                                                                              Si     Fe     Ca  Mg   K   Na  Li   Mo   Cr  Cu                        ______________________________________                                        WPPM   50     4      7   180  50  80  <1   10   2   4                         Detect-                                                                       ed                                                                            ______________________________________                                    

Magnesium oxide, typically added to alumina as a sintering aid to obtaina PCA body of densities greater than 3.97 g/cc, has been found toprevent the conversion of PCA to sapphire if present in sufficientquantity. Thus, steps must be taken to lower the magnesium oxide contentof the fully dense doped PCA body prior to conversion to sapphire. Thelevel to which magnesium oxide must be lowered can depend on the typeand amount of the dopant added. In some cases it has been found that itis necessary to reduce the magnesium content to as low as 50 wppm priorto converting the material to sapphire. Magnesium oxide content wasdetermined using Inductively Coupled Plasma (ICP) analysis. Thoseskilled in the art know that magnesium oxide can be driven out of a PCAbody by heating the body in a vacuum, dry hydrogen, or inert gascontaining atmosphere to temperatures above 1600° C. In the process ofthis invention, this was accomplished by heating the doped Lucalox®brand PCA in an electric resistance furnace to temperatures of 1880° C.for approximately one to nine hours, depending on the part size dopanttype, and dopant level, in an atmosphere of dry hydrogen having a dewpoint below 0° C. Times required to drive magnesium oxide from analumina body will vary based on starting magnesium oxide content,furnace temperature, furnace atmosphere, and part dimension. Care mustbe taken during the magnesium oxide volatilization to avoid heating thematerial for too long as this can result in average grain sizes greaterthan 100 μm and/or anomalous grain growth.

In accordance with this invention, the bisque-fired starting material isdoped to a selected concentration with a conversion-enhancing dopant. Asused herein, "conversion-enhancing dopant" refers to a dopant that, whenintroduced into the polycrystalline material at a concentration asdescribed herein, reduces the time necessary to effect the solid stateconversion of the polycrystalline material to a single crystal materialduring the heating process as discussed below. The selectedconcentration of the dopant is less than the solid solubility level ofthe dopant in the polycrystalline material. The selected concentrationof conversion-enhancing dopant thus is less than a concentration thatwill result in the formation of a second crystalline phase in thepolycrystalline material. In doping the polycrystalline material, thedopant is typically dispersed in the polycrystalline material such thatthe concentration of the dopant is substantially homogeneous throughoutthe polycrystalline material. Based on dopant levels that have beeninvestigated to date, it has been observed that conversion rate (fromPCA to sapphire) generally increases as the concentration of the dopantis increased.

For example, a PCA body doped at a selected concentration with chromium,or alternatively with gallium, or alternatively with titanium, has beenshown to convert to a single crystal structure body during heating inless time than a similarly-sized undoped PCA body, as set forth ingreater detail below. Conversion-enhancing dopants for alumina typicallycomprise cations having a +3 valence and that exhibit appreciable solidsolubility in Al₂ O₃. "Appreciable solid solubility", as used herein,refers to a material that, when mixed with Al₂ O₃ at a level below itsrespective solid solubility limit results in a PCA-to-sapphireconversion rate of practical significance. "Practical significance", asused herein, refers to conversion rates on the order of centimeters perhour, which conversion rates provide sufficient efficiency to make theproduction of single crystal with that dopant commercially feasible.Typically such a solid solubility of materials exhibiting thesecharacteristics is about 50 wppm or more. For example, the solidsolubility levels in Al₂ O₃ is as follows for the notedconversion-enhancing dopants:

    ______________________________________                                        Chromium         100% solid solubility                                        Titanium         ˜50-100 wppm                                           Gallium          20-30% by weight                                             ______________________________________                                    

Other +3 valence cations that may, at a selected concentration, exhibitconversion enhancing properties in alumina include cerium, samarium,lanthanum and vanadium.

In one embodiment of this invention, a bisque-fired polycrystallinematerial is doped, for example, by immersing the bisque-fired materialin a liquid solution of the selected conversion-enhancing dopant. Thedoping solution typically comprises deionized water with dopantdissolved therein to provide a predetermined concentration of the dopingion. Infiltration of the dopant into the bisque-fired material isfurther induced, for example, by disposing the bisque-fired material,along with the doping solution in which it is immersed, in a vacuumchamber and drawing a selected vacuum on the chamber for a selectedtime. After immersion (and the exposure to the vacuum chamber, asappropriate) the doped bisque-fired material is removed and allowed todry, typically in air, after which it is ready for heating in a furnaceto effect the solid state conversion process to a single crystalmaterial.

The bisque-fired body doped to have the selected concentration ofconversion-enhancing dopant is then heated to a temperature not greaterthan the melting point of the material comprising the ceramic body.Additionally, the temperature is typically greater than one-half themelting point of the material comprising the ceramic body. For ceramicbodies comprising alumina as described herein, the PCA body is heated toa temperature greater than 1100° C. but less than 2050° C., the meltingpoint of alumina. No bulk melting of the ceramic is observed during theprocess of converting the alumina to sapphire, and growth of the singlecrystal structure is typically initiated from more than one site on thebody. Details of the solid state thermal conversion process arecontained in co-pending application entitled "Solid State ThermalConversion of Polycrystalline Alumina to Sapphire," Ser. No. 08/126,954,incorporated herein by reference.

In one example of the process of this invention, bisque-fired aluminatubing comprising Lucalox® brand PCA, as described above and being about220 mm long, with about a 5 mm outer diameter and a 0.5 mm wallthickness, was doped with chromium in accordance with the followingprocedure: A doping solution was prepared by dissolving 0.918 grams ofchromium (III) nitrate (Cr(NO₃)₃.gH₂ O) in 1 liter of deionized water,which provides a Cr doping ion concentration of approximately 120 wppm.Pieces of presintered alumina tubing, described above, were immersed inabout 400 ml of the doping solution. The doping solution with thebisque-fired alumina immersed therein was then placed in a vacuumchamber; a vacuum of about 27" Hg was drawn on the vacuum chamber andmaintained for about 30 minutes, after which the doped bisque-firedalumina tubes were removed from the vacuum chamber and the dopingsolution and allowed to air dry. The resulting concentration of chromiumdopant (the conversion-enhancing dopant)in the bisque-fired aluminatubes was about 300 wppm. A similar doping procedure was used to dopebisque-fired alumina tubes having a large outer diameter (8.8 mm).

Fabrication of sapphire from the doped, bisque-fired material involvesthree basic stages, namely: sintering the material to achieve a densebody (i.e., densities greater than 3.90 g/cc); reducing the magnesiumoxide concentration (magnesium oxide being a conversion impedingimpurity) in the body below a level that impedes conversion of thematerial to single crystal; and heating the body to convert thepolycrystalline material to a single crystal. In one embodiment of thisinvention, these three stages are accomplished in multiple heatingcycles (that is, raising the temperature of the polycrystalline materialin separate processes); the three stages can be accomplished in threerespective heating cycles, or alternatively in three or more heatingcycles. In another embodiment of the invention, the three stages areaccomplished in a single continuous heating cycle in which all threestages occur without removal of the doped polycrystalline body from thefurnace used for the heating; in the single heating process there is noclear distinction between the occurrence of the three stages.

In one example of the process of this invention, the three stages of theconversion of PCA to sapphire were accomplished in a single continuousheating process as the three stages could be accomplished usingidentical furnace conditions. The doped, bisque-fired material waspassed in a continuous manner through the hot zone of an electricresistance furnace having a temperature of about 1880° C. and anatmosphere of flowing dry hydrogen having a dew point less than about 0°C. In another example of the process of this invention, the three stagesof sapphire fabrication (that is, conversion of the PCA to sapphire)were accomplished using multiple shorter time passes through a furnacehaving similar conditions to those described above for the continuousheating process. The total time in the hot zone (achieved either in onecontinuous pass through the furnace or through multiple shorter timepasses) necessary to reach 100% conversion is a function of the dopantlevel, dopant type, starting magnesia level, and body geometry (that is,thicker body dimensions translate into longer times required to reduceMgO content below the level which prevents conversion).

In one example of the process of this invention, alumina tubes dopedwith chromium (as described above) were exposed to heating in a furnacefor the times noted resulting in conversion to sapphire as noted inTable 2:

                  TABLE 2                                                         ______________________________________                                        TIME IN HOT ZONE OF FURNACE                                                   (5.0 mm outer diameter tubes)                                                 Sample   3 HRS     6 HRS     9 HRS   12 HRS                                   ______________________________________                                        Cr-Doped ˜8% ˜68%                                                                              100%                                             Tube (5.0 mm                                                                           conversion                                                                              conversion                                                                              conversion                                       OD)                                                                           Control Tube                                                                           No        ˜5% --      ˜83%                               (5.0 mm OD)                                                                            sapphire  conversion        conversion                               ______________________________________                                        TIME IN HOT ZONE OF FURNACE                                                   (8.8 mm outer diameter tubes)                                                 Sample   6 HRS     9 HRS     12 HRS  15 HRS                                   ______________________________________                                        Cr-Doped ˜5% ˜47%                                                                              ˜90%                                                                            100 %                                    Tube (8.8 mm                                                                           conversion                                                                              conversion                                                                              conversion                                                                            conversion                               OD)                                                                           Control Tube                                                                           No        No        No      No                                       (8.8 mm OD)                                                                            sapphire  sapphire  sapphire                                                                              sapphire                                 ______________________________________                                    

Examination of the data presented in Table 2 shows that the tubes dopedwith chromium converted to sapphire in markedly less time than thecontrol (undoped) tubes. The apparent effect of body geometry is alsoevident as the larger, thicker-walled 8.8 mm OD tubes required longerheating times to convert, most probably as a result of the longer timesnecessary for the removal of MgO in the thicker parts.

FIG. 1 presents data illustrating the relative effect on PCA-sapphireconversion rate of the following dopants with respect to an undopedcontrol sample: titanium; chromium; and gallium.

In an alternative embodiment of the present invention, variations of theprocess of this invention can also be used to make a composite material,that is, a body having both single crystal structure and polycrystallinestructure regions. Such a composite body is formed by selectively dopingportions of a polycrystalline bisque-fired ceramic material and thenheating the material as described above to effect conversion of thedoped portions of the polycrystalline material. For example, a spiraldesign was painted down the length of a bisque-fired alumina tube. Thedoping solution used to paint the design comprised Cr(NO₃)₃.9H₂ Odissolved in deionized water to yield a solution having a Cr ionconcentration of about 120 wppm. The partially-doped bisque firedalumina tube was heat treated according to the process of the presentinvention described above. Following the heat treatment, the areapainted with the Cr dopant solution was converted into sapphire and theunpainted region of the alumina tube remained polycrystalline. Nocracking along the interlace between the two regions was observed.

Alternatively, a composite material is formed by selectively doping afirst portion of the body with a first concentration of a selectedconversion-enhancing dopant and doping a second portion of the body witha second concentration of the conversion-enhancing dopant, the secondconcentration being greater than the solid solubility limit of theselected conversion-enhancing dopant so as to cause the precipitation ofa second phase along the grain boundaries of the second portion of thebody. The first and second portions of the polycrystalline body are thenheat treated in accordance with this invention as described above so asto convert the first portion of the body to a single crystal structure.The second doped portion of the body, that is, the areas doped to have adopant level above the solid solubility limit, do not convert due to thepresence of the second phase at the boundary of this portion of thebody. The first portion of the body, that is, the areas doped at levelslower than the solid solubility limit, convert to a single crystal.

The solid state conversion process in accordance with this inventionallows for simple or complex ceramic shapes to be fabricated usingstandard polycrystalline forming techniques and then converted into asingle crystal body without melting the body. Thus, the single crystalbody maintains the shape of the polycrystalline precursor, enabling thefabrication of a great diversity of shapes that are not feasible tofabricate using conventional melt drawing or floating zone techniques.Common shapes of alumina materials that beneficially comprise sapphireinclude fibers (as might be used in fiber reinforced material) or tubes(such as those described above in the demonstration of the method ofthis invention). Single-crystal lamp parts made from the above-mentionedtubes typically exhibit superior light transmittance characteristics(both total transmittance and forward diffuse transmittance), resultingin increases in lamp efficiency of about 10-15%.

Sapphire tubes produced according to the method of this invention arereadily formed in the shape of tubes that are adapted for use as arctubes in high pressure sodium (HPS) lamps. FIG. 2 illustrates an exampleof an HPS lamp 10 comprising an arc tube 20 fabricated in accordancewith this invention. Arc tube 20 is hollow, having an interior surface22 and an exterior surface 24, and contains the high pressure sodiumused for lamp operation. The total and forward diffuse visible lighttransmittance of a ceramic arc tube fabricated in accordance with thisinvention provides improved HPS lamp efficiency over that of PCA arctubes currently in use.

Further, the method of this invention enables tubes (or other shapes ofobjects) to be made that comprise a first portion having a singlecrystal structure and a second portion having a polycrystallinestructure. Thus it is possible to fabricate an arc tube 20 having aninterior surface 22 comprising sapphire and an exterior surface 24comprising polycrystalline alumina; one advantage of this tube structureis that the interior sapphire surface provides better resistance tosodium attack than does a polycrystalline surface, and thepolycrystalline material on the outer surface of the tube provides goodstrength for the tube. Similarly, if desired, the arc tube can befabricated so that interior surface 22 comprises polycrystalline aluminaand exterior surface 24 comprises sapphire.

Sapphire produced according to the process of this invention isdistinguished from sapphire fabricated using melt drawing techniques bythe combination of a random pore arrangement and a unique surfacetopography in the form of slight undulations having high points atapproximately the midpoint of where each PCA grain was located prior tothe conversion to sapphire and depressed areas corresponding to wherethe grain boundaries were located prior to conversion. By contrast,sapphire fabricated by many melt-drawing techniques typically exhibits alinear arrangement of porosity resulting from bubbles produced duringthe drawing process.

While particular embodiments of the present invention are describedherein, it is understood that various other modifications will beapparent to and can be readily made by those skilled in the art withoutdeparting from the scope and spirit of the invention in its broaderaspects. Accordingly, it is not intended that the scope of the claimsappended hereto be limited to the description set forth above but ratherthat the claims be construed as encompassing all of the features ofpatentable novelty which reside in the present invention, including allfeatures which would be treated as equivalents thereof by those skilledin the art to which the invention pertains.

For example, suitable bisque-fired aluminas other than those used in theproduction of Lucalox® brand PCA can be used in the practice of thisinvention. Such materials can be prepared, for example, from aluminapowders in accordance with the known methods, with suitable selecteddopants introduced at the bisque-fired stage. Example of such knownmethods of processing alumina powders include the processes disclosed inU.S. Pat. Nos. 3,026,210 and 4,1150,317, both of which are assigned tothe assignee herein and incorporated by reference. Alternatively, thedopant material can be introduced into the raw material used to producethe polycrystalline body in the same manner that magnesium is introducedinto alumina in accordance with the procedures disclosed in U.S. Pat.Nos. 3,026,210 and 4,1150,317. Starting materials fabricated in eitherof the above manners, or other similarly efficacious manners apparent tothose skilled in the art, should provide suitable starting materials forthis process provided the materials meet the requirements of purity,density, grain size, and grain structure previously described.

This process similarly is not restricted to the conversion of PCA tosapphire; any polycrystalline body doped with a selected concentrationof an appropriate (that is, a dopant observed to enhance the conversionrate of the particular material in question) conversion-enhancingdopant, if sufficiently pure and pore-free, can be convened to a singlecrystal form by heating below the melting temperature of the body in anappropriate atmosphere in accordance with the process set out above.

What is claimed is:
 1. A solid state process for convening apolycrystalline ceramic body doped with a selected conversion-enhancingdopant at a selected concentration to a single crystal body comprisingthe steps of:doping at least a portion of the polycrystalline ceramicmaterial comprising said ceramic body with a conversion-enhancing dopantto a selected concentration; and heating the doped polycrystalline bodyto a selected temperature to convert the portion of the polycrystallinebody doped to said selected concentration to a single crystal structure,said selected temperature being above one-half of the meltingtemperature of said doped ceramic material but below the meltingtemperature of said doped ceramic material.
 2. A process according toclaim 1 wherein the step of doping the polycrystalline ceramic materialcomprising said ceramic body comprises doping a bisque-fired precursorof said polycrystalline ceramic body with said conversion-enhancingdopant to said selected concentration.
 3. A process according to claim 2wherein the step of doping said bisque-fired precursor comprisesimmersing said bisque-fired precursor in a liquid solution having aconcentration necessary to achieve said conversion of saidconversion-enhancing dopant and allowing said dopant to penetrate intosaid bisque-fired precursor to said selected concentration of dopant insaid bisque-fired precursor.
 4. A process according to claim 1 whereinthe step of doping the polycrystalline ceramic material-comprising saidceramic body comprises the steps of doping a starting powder with aselected level of said selected conversion-enhancing dopant; andprocessing said starting powder to form said ceramic body.
 5. A processaccording to claim 1 wherein said selected concentration of saidconversion-enhancing dopant is less then that concentration which willresult in the formation of a second crystalline phase in saidpolycrystalline material.
 6. A process according to claim 1 wherein thestep of doping said polycrystalline material further comprises the stepsof dispersing said conversion-enhancing dopant homogeneously in saidpolycrystalline ceramic body.
 7. A process according to claim 1 whereinthe step of heating said doped polycrystalline body further comprisesthe steps of:sintering said polycrystalline body to removeconversion-impeding impurities; and heat treating said polycrystallinebody to convert said dense body to a single crystal.
 8. A processaccording the claim 7 wherein the steps of sintering saidpolycrystalline body, heat treating said polycrystalline body to removeconversion-impeding impurities, and heat treating said polycrystallinebody to convert the body to single crystal comprise multiple heatingcycles.
 9. A process according the claim 7 wherein the steps ofsintering said polycrystalline body, heat treating said polycrystallinebody to remove conversion-impeding impurities, and heat treating saidpolycrystalline body to convert the body to single crystal compriserespective distinct heating cycles.
 10. A process according the claim 7wherein the steps of sintering said polycrystalline body, heat treatingsaid polycrystalline body to remove conversion-impeding impurities, andheat treating said polycrystalline body to convert the body to singlecrystal comprise a single continuous heating cycle.
 11. A processaccording to claim 1 wherein said polycrystalline body is in the shapeof a fiber.
 12. A process according to claim 1 wherein saidpolycrystalline body is in the shape of a tube.
 13. The process of claim1 wherein the step of doping at least a portion of said ceramic bodywith a conversion-enhancing dopant to a selected concentration furthercomprises doping said a first portion of said ceramic body and notdoping a second portion of said ceramic body such that the step ofheating said body produces a composite material body comprising a singlecrystal structure in said first portion and a polycrystalline structurein said second portion.
 14. The process of claim 1 wherein the step ofdoping at least a portion of said ceramic body with aconversion-enhancing dopant to a selected concentration furthercomprises doping a first portion of said ceramic body with said dopantto said selected concentration and doping a second portion of saidceramic body with said dopant to a second phase level concentration;saidsecond phase level concentration being greater than the solid solubilitylevel of said dopant in polycrystalline ceramic material such that asecond phase is formed along grain boundaries of said second portion ofsaid doped body so as to inhibit the conversion process; wherein thestep of heating of said body produces a composite material body in whichsaid first portion comprises a single crystal structure and said secondportion comprises a polycrystalline structure.